1. Field of the Invention
The present invention relates to a method for heating effectively a steel slab for a grain-oriented electrical steel sheet in a walking-beam type heating furnace, and particularly a method for operating such a heating furnace to achieve effective heating of the slab.
Grain-oriented electrical steel sheet, for example, a cube-on-edge oriented electrical steel sheet, has a texture in which the (110) plane of the grains is oriented pallarel to the rolling plane and the [001] plane is oriented parallel to the rolling direction. Grain-oriented electrical steel sheet having such a crystallographic orientation of (100)[001] is characterized by excellent magnetic properties in the rolling direction, and has been widely used in various industries as iron cores for transformers, a large rotator, etc. to take advantage of this characteristic.
2. Description of Prior Art
In the production method for a grain-oriented electrical steel sheet, the following process, for example, has been commonly practised as a conventional process.
A hot rolled steel sheet normally about 2.5 mm thick is prepared by melting a suitable steel composition, casting a refined steel ingot or slab from the molten steel, and hot rolling the ingot or slab with necessary heat treatments, and the hot rolled steel sheet is then subjected to acid pickling, one step cold rolling, or two-or more-step cold rolling including intermediate annealing to obtain a cold rolled steel sheet having the final thickness, sheet is then subjected to decarburization and final high-temperature annealing.
In the final high-temperature annealing of the above conventional process, the properties required for the grain-oriented electrical steel sheet are obtained by development of secondary recrystallization grains having (110)[001] orientation. In this case, precipitates produced by trace elements contained in the steel play an important role. As the precipitates, MnS, AlN, MnSe, etc. have been widely known, and what is most important is to form precipitates as fine as possible in the hot rolled steel sheet. For this purpose, it has been considered to be essential that the steel slab prior to the hot rolling be soaked to a temperature high enough to dissolve impurities during the slab heating step, and that appropriate cooling conditions be used so as to assure reprecipitation of the impurities as fine precipitates. Regarding the slab heating for dissolving the impurities as a pretreatment for formation of fine precipitates, a slab heating temperature ranging from 1260.degree. to 1400.degree. C has been commonly used as taught U.S. Pat. No. 2,599,340.
However, when a higher temperature within the above slab heating temperature range is applied, abnormal growth of the slab grains is caused, and this abnormal grain growth causes deterioration of the magnetic properties.
Therefore, it is almost impossible to produce a grain-oriented electrical steel sheet having a high degree of stabilized magnetic properties, unless heating conditions are used which avoid the above drawback. Thus, in the treating of a slab for a grain-oriented electrical steel sheet, it is important to heat the slab uniformly so as to assure that the lowest temperature portion in the slab is not lower than the dissolution temperature of the impurities and the highest temperature portion in the slab is lower than the temperature which causes the abnormal grain growth.
Conventionally, as the slab heating furnace for the above purpose, a pusher-type heating furnace as shown in FIG. 1 has been widely used, although a walking-beam type heating furnace as shown in FIG. 2 has also come to be used.
In the operation of the pusher-type heating furnace, the slab 1 is pushed by a pusher 3 and slid onto a watercooled skid 2. During the sliding onto the skid, skid marks are formed on the under surface of the slab. Therefore, where uniform heat soaking is required as in the heating of a slab for a grain-oriented electrical steel sheet, such operating techniques are used as making the traveling time of the slab along the floor 4 of the soaking furnace longer, or the position and arrangement of the skid rails are changed so as to make the skid rails contact with the slab under surface only at one position, as disclosed in Japanese Patent Publication Sho 38-15425, Japanese Utility Model Publications Sho 42-18766 and Sho 41-19210.
Thus, the pusher-type heating furnace has the defect that considerable surface damage is caused to the slab under surface during the sliding of the slab on the skid rails or on the bottom brick work of the soaking furnace. This defect is serious in the heating of a slab for a grain-oriented electrical steel sheet, where the slab is heated at a high temperature as compared with an ordinary steel slab in order to obtain excellent magnetic properties, and the hot strength of the slab is considerably lower than that of an ordinary steel slab due to the high silicon content of 2.5 to 4%. As a result, the commercial value of the product will be completely lost due to the damage to the under surface. Further in case of a slab for grain-oriented silicon steel sheet, as well as an ordinary steel slab, a molten scale (commonly called "slag") mainly composed of iron oxides (FeO, Fe.sub.2 O.sub.3, SiO.sub.2) is formed by the reaction between the furnace atmosphere and the slab surface, and this scale accumulates irregularly on the skid rails or on the bottom floor of the soaking pit so that a subsequent slab may be caused to run over a preceding slab in what is commonly called a slab overlapping phenomenon which hinders the furnace operation.
As one solution to the problems of surface damage to the under surface of the slab and the irregular scale accumulation in the pusher-type heating furnace, it has been proposed that the slab heating be concentrated only on the upper surface and the temperature of the under (lower) surface be kept relatively low until the slab enters the soaking zone so as to reduce the surface damage.
However, this solution is not very effective to prevent the back surface damage caused by the sliding on the soaking zone floor, and has a drawback that the temperature of the slab upper surface very often becomes extremely high during the heating because the surface temperature of the lower or under surface is raised by the heat retained by the slab upper surface after the slab enters the soaking zone, resulting often in abnormal grain growth and hence abnormal magnetic properties.
As described above, the method of heating a slab for a grain-oriented electrical steel sheet in a conventional pusher-type heating furnace does not achieve completely satisfactory results with respect to both the surface defects and the magnetic properties, and a suitable solution of these problems has long been sought.
Meanwhile the walking-beam type heating furnace which has just come to be used for the slab heating has a remarkable efficacy for preventing under surface damage which is unavoidable in the pusher-type heating furnace, because a transfer beam 5 and a fixed beam 6 are provided, and the slab 1 is lifted up and transferred a certain constant distance by the transfer beam as shown in FIG. 2 and FIG. 3. But the walking-beam type furnace has not yet been adopted for heating a slab for grain-oriented electrical steel sheet for the following reasons.
(1) Because the transfer of the slab in the walking-beam type treating furnace is performed by the combination of the driving of the transfer beam and the fixed beam, the slab is curved or bent by the shock given to the slab when the slab is transferred from the transfer beam onto the fixed beam, or when it is taken up onto the transfer beam from the fixed beam, resulting in difficulties in the slab transfer within the furnace or the slab transfer by means of a table roller.
(2) The scale accumulation on the beams caused by the high temperature heating of the slab causes damage to the slab under surface and the slab is difficult to extract due to its having been turned at an angle to the transfer direction because of scale accumulation, and when the distance between beams is made small in order to reduce the problem (1) the apparent diameter of the beams is increased by the scale accumulation and thus the beams interfere with each other, hindering smooth driving of the beams.